A lithium secondary battery is generally assembled with and includes an anode as a negative electrode, a cathode as a positive electrode, and a separator interposed between the anode and the cathode, in which the separator located between the two electrodes of the lithium secondary battery is a subsidiary material to prevent the anode and the cathode from being in direct contact with each other and from being internally short-circuited, to thus play an important role of improving safety of the secondary battery as well as providing ion channels in the secondary battery.
In the case of a conventional battery manufactured by using a polyolefin-based separator, a phenomenon that the two electrodes and the separator are not adhered closely to each other and are seceded from each other occurs frequently. Accordingly, the lithium ion delivery is not effectively achieved through pores of the separator, and thus the battery performance is lowered.
Further, the conventional separator is made of a chemically stable material that does not cause decomposition and reaction when being exposed to an oxidizing and reducing atmosphere inside the battery, for example, a fluorine-based polymer, in which the mechanical strengths of these substrates are not satisfactory, to thus cause problems such as peeling and breakage of the separator during an assembly process of the battery, and to thereby cause deterioration of safety such as internal short-circuit of the battery. Additionally, to promote heat-resistant performance or high dielectric constant of separators, inorganic particles are coated on the separators. However, the inorganic particles are desorbed due to a low binding capability between the separator and the inorganic particles, to thus fail to realize a desired effect.
Meanwhile, when a large-capacity secondary battery with high energy density should have a relatively high operating temperature range, and continue to be used at a high rate charge-discharge state, the temperature of the battery rises up. Therefore, separators that are used for these batteries require higher heat resistance and heat stability than those required in typical separators. In addition, the large-capacity secondary battery with high energy density should possess battery characteristics such as high ion conductivity so as to respond to rapid charge-discharge and low temperature.
The separator is placed between the anode and the cathode of a battery to perform an isolation function. The separator maintains an electrolytic solution to thus provide an ionic conduction pathway. The separator has a shutdown function of blocking the pores by melting part of the separator to block electric current if the battery temperature rises up too much.
When the separator is melted as the temperature gets higher, a big hole is created to thus cause a short circuit to occur between the anode and the cathode. The temperature is called a short-circuit temperature. Generally, the separator should have a lower shutdown temperature and a higher short-circuit temperature.
Therefore, it is very important for the secondary battery to have both a shutdown function and a heat-resistance performance in order to achieve a high-energy density and large-area secondary battery. In other words, it is required that the separator should have an excellent heat-resistance performance to thus cause small thermal shrinkage and an excellent cycling performance due to a high ionic conductivity.
It is very deficient to use an existing lithium-ion secondary battery using a polyolefin separator and a liquid electrolyte or an existing lithium-ion polymer battery using a polymer electrolyte that is obtained by coating a gel polymer on a gel polymer electrolyte or a polyolefin separator, for a high-energy density and large-capacity secondary battery in terms of the heat-resistance. Therefore, the heat-resistance performance that is required for a high-capacity and large-area secondary battery for automobiles does not meet the safety requirements.
Korean Patent Application Publication No. 2008-13209 discloses a separator having a heat-resistance and ultra-fine fibrous layer as the separator that is formed by coating a fibrous layer on one surface or both surfaces of a porous film, in which the fibrous layer includes: a fibrous material that is formed by electrospinning a heat-resistance polymer material whose melting point is not less than 180° C. or having no melting point; and a fibrous material that is formed by electrospinning a swellable polymer material on an electrolytic solution.
In the Korean Patent Application Publication No. 2008-13209, since a conventional polyolefin-based porous film used as a separator is used as a central substrate, the fibrous layer laminated on the polyolefin-based porous film having a low porosity may have a limitation that any layer of a multilayer structure cannot have excellent properties of a porous web obtained by an electrospinning method, particularly, ion conductivity due to a high porosity.
In addition, Korean Patent Application Publication No. 2004-108525 discloses a composite film for an electrochemical device in which the composite film uniformly absorbs an electrolyte solution, to thus greatly improve performance of a battery, and in which the composite film has an excellent mechanical strength and a satisfactory binding force with electrodes, to thus increase a battery manufacturing process speed.
In the Korean Patent Application Publication No. 2004-108525, the composite film has a structure that a porous film of a polymer web form is laminated on one surface and/or both surfaces of a polyolefin-based microporous film used as a strength support, in which the polyolefin-based microporous film has an average pore size of 0.005˜3 μm, a porosity of 30˜80%, a tensile strength of 700 kg/cm2 or more in a mechanical direction thereof, a transverse tensile strength of 150 kg/cm2 or more, and a thickness of 5˜50 μm.
In particular, in an embodiment of the Korean Patent Application Publication No. 2004-108525, a composite film having an entire porosity of 58% is obtained when web-shaped porous films of a porosity of 55% and a porosity of 80% of a polyolefin-based microporous PP (polypropylene) film are laminated to obtain a three-layer structure, and a composite film having an entire porosity of 45% is obtained when a web-shaped porous film is formed on a polyolefin-based microporous PE (polyethylene) film of a porosity of 43% by an electrospinning method to thus laminate a three-layer structure.
Thus, the composite film of the Korean Patent Application Publication No. 2004-108525 has porosity depending on that of the polyolefin-based porous film since the porosity of the polyolefin-based porous film is greatly lower than that of the web-shaped porous film, resulting in falling of ionic mobility characteristics. That is, the composite film does not use properties of the web-shaped porous film having a high porosity at maximum.